A methodology for verification of radiotherapy dose calculation

Object. A methodology for dosimetric verification of radiation therapy plans was developed and implemented. Dosimetric accuracy of clinically active intensity-modulated radiotherapy (IMRT) and intensity-modulated radiosurgery (IMRS) programs was assessed using this methodology. Methods. The methodology included several dosimetric tasks that were performed to assess the dosimetric accuracy of a treatment plan. Absolute dosimetry of the composite plan was performed using an ionization chamber. Film dosimetry was performed for each individual field and for the multifield composite plan. Calculated dose distributions and film measurements were compared using software developed for the specific tasks. Two-dimensional maps of gamma index, dose difference, and distance-to-agreement were calculated and displayed. To date, good agreement between measurements and calculations has been observed in 160 clinical IMRT and IMRS plans. The largest observed absolute dose disagreement was -4.79%. The mean absolute dose difference was 0.26%, with a standard deviation of 1.75%. The authors specify a 3% dose difference and 3-mm distance as the scaling acceptability criteria for the gamma index calculations of the film measurement analysis. The planning and delivery system in clinical use has proven consistently to satisfy these criteria. Conclusions. The dosimetric verification methods and the software tools developed were both quantitative and clinically practical. The measurements and the analysis demonstrated that the IMRT and IMRS planning and delivery system in use was sufficiently accurate for highly conformal treatments.